The present invention relates to an injection moulding apparatus, and in particular to a valve gate drop assembly for an injection moulding apparatus. Specifically, the present invention relates to reduction in the deflection of a valve pin in the valve gate drop assembly during movement of the manifold resulting from thermal expansion and contraction of the manifold.
One method for the manufacture of articles from thermoplastic polymers involves injection of molten polymer through a nozzle into a mould. The molten polymer is fed from a source of molten polymer, typically an extruder, into the inlet of a manifold and then transferred through one or more channels in the manifold towards nozzle assemblies. Each nozzle assembly is normally oriented at a substantial angle to the manifold, most commonly at about 90xc2x0, and has a channel for molten polymer communicating with a channel in the manifold. Each nozzle assembly also includes a nozzle gate connected to the cavity of the mould for the article that is to be manufactured.
During the moulding cycle, molten polymer entering a nozzle assembly from the manifold is injected through its nozzle gate into the mould. After the mould has been filled with molten polymer, the mould is cooled to solidify the polymer so that when the mould is opened to remove the article, the moulded article retains its shape and does not undergo distortion.
The channels in the manifold and nozzle assemblies contain molten polymer under pressure even during the steps in the injection moulding cycle when molten polymer is not being injected into moulds. Thus, it is necessary to prevent flow of molten polymer through the nozzle gates at certain stages in the moulding cycle. It is also necessary to control the amount of molten polymer that flows into the moulds during the injection stage of the moulding cycle. This is achieved by use of a valve gate drop assembly for each nozzle assembly. Each valve gate drop assembly has a valve pin extending from a valve pin cylinder. The valve pin passes through the manifold and extends into the nozzle assembly terminating at the nozzle gate. Control of flow of molten polymer is achieved by raising and lowering the valve pin. Retracting the valve pin from the nozzle gate permits flow of molten polymer into the mould while re-inserting the valve pin into the nozzle gate prevents further flow of molten polymer into the mould. The movement of the valve pin is controlled by the valve pin cylinder, which may be, for example, a pneumatic or hydraulic system that raises and lowers the valve pin.
The intermittent flow of molten polymer through the manifold and nozzle assemblies, the cooling of the moulds to effect removal of the moulded articles and the subsequent re-heating and control of the temperature for injection of molten polymer, results in temperature changes in the valve gate drop assemblies, the nozzle assemblies, and the manifold. The required temperatures and control of such temperatures are achieved using heaters in the manifold and/or nozzle assemblies.
The heating, cooling and temperature control of the manifold and nozzle assemblies, including start up and shut down of the injection moulding apparatus, results in some thermal expansion and contraction of the manifold and nozzle assemblies. Relative changes in position of the manifold and nozzle assemblies apply stress to the valve pins, which pass through the manifold and extend into the nozzle assemblies, with the consequence that the valve pins tend to be forced away from alignment with the nozzle gates i.e. be deflected. This can lead to bending and damage of the valve pins which in turn can result in damage to the nozzle gates. Bending of the valve pins and damage to the nozzle gates can lead to loss of control of flow of molten polymer through the nozzle gates e.g. by poor seating of the valve pins in the nozzle gates and/or changes in timing of closing of the nozzle gates. This of course can lead to process problems such as polymer drool at the nozzle gates or inconsistent injection of molten polymer into the moulds.
The nozzle assemblies, valve gate drop assemblies and manifold may be bolted together, so that there is no relative movement in the positions of the valve pin cylinders, manifold and nozzle assemblies on change in temperature. However, this can lead to stress in other parts of the injection moulding apparatus and creates increased heat loss. An alternative is to permit the manifold to float in position with respect to both the valve pin cylinders and the nozzle assemblies. In other words, the manifold is not bolted to the valve pin cylinders or to the nozzle assemblies, in which case there is a tendency for the valve pins to be deflected from alignment and become distorted or bent as the manifold undergoes thermal expansion and contraction.
It is therefore an object of the present invention to provide a novel valve gate drop assembly for an injection moulding apparatus and an injection moulding apparatus incorporating the same.
The present invention provides a valve gate drop assembly for an injection moulding apparatus in which deflection of the valve pin from alignment may be reduced during movement of the manifold relative to the nozzle assembly resulting from thermal expansion and contraction.
Accordingly, one aspect of the present invention provides an injection moulding apparatus comprising:
a manifold having a channel receiving an injection of molten polymer and delivering said molten polymer to a nozzle assembly, said nozzle assembly including a nozzle gate adapted to be connected to a mould to deliver molten polymer thereto and;
a valve gate drop assembly having a valve pin extending from a valve pin cylinder through said manifold and into said nozzle assembly, said valve pin being aligned with said nozzle gate and adapted for control of flow of molten polymer through said nozzle gate by movement into and away from said nozzle gate, movement of said valve pin being controlled by said valve pin cylinder, said manifold undergoing a change in position relative to the positions of the valve pin cylinder and nozzle assembly on thermal expansion and contraction of the manifold, said valve pin passing through an eccentric bushing in said manifold, said eccentric bushing rotating on change of position of the manifold generally to maintain alignment of said valve pin with said nozzle gate thereby to reduce deflection of said valve pin caused by said change of position of the manifold.
According to another aspect of the present invention there is provided in an injection moulding apparatus including a manifold having a channel feeding molten polymer to a nozzle assembly for subsequent delivery to a mould, said manifold and nozzle assembly undergoing relative movement as a result of thermal expansion and contraction of said manifold, a valve gate assembly comprising:
a valve pin extending through said manifold and into said nozzle assembly;
a drive acting on said valve pin to move said valve pin between an extended position where said valve pin seals said nozzle assembly and a retracted position where molten polymer can flow from said nozzle assembly to said mould; and
a rotatable element accommodated by said manifold and through which said valve pin passes, said rotatable element rotating within said manifold upon movement of said manifold as a result of thermal expansion and contraction generally to maintain alignment of said valve pin and said nozzle assembly thereby to reduce deflection of said valve pin.
Preferably the rotatable element is an eccentric bushing that rotates up to about 20xc2x0 during expansion and contraction of the manifold. It is also preferred that the drive includes a valve pin cylinder having a piston acting on the valve pin.
The present invention provides advantages in that since the nozzle assembly is allowed to float with respect to the manifold while inhibiting deflection of the valve pin, the amount of stress applied to components of the injection moulding apparatus is reduced while maintaining adequate process control.